Sea urchin genetic material will be cloned as recombinant DNA in bacterial plasmids and sequences expressed as messenger RNA in sea urchin cells selected. The selection procedure will employ the colony hybridization technique of reacting fixed colonies of bacteria carrying the recombinant plasmids to mRNA isolated from sea urchin eggs and embryos and labeled by various in vivo and in vitro reactions. By reacting the colonies serially to labeled mRNA from different stages, sequences which are expressed as mRNA at one stage but not at another will be selected. The nature of these mRNA sequences and those adjacent to them will be studied by various procedures. Their association with repetitive and single-copy sequences and their expression as hnRNA will be especially studied.